The invention relates to a rotary damper for damping a rotation of a driven gear engaging a gear or a rack.
A rotary damper is formed of a housing; a viscous fluid filled inside the housing; a rotor disposed in the housing and having a shaft part partially protruding from the housing and a resistance part provided on the shaft part for moving within the viscous fluid inside the housing; and a seal member for sealing between the shaft part of the rotor and the housing to prevent leakage of the viscous fluid. A driven gear is attached to the shaft part protruding from the housing (refer to Japanese Patent Publication (Kokai) No. 04-34015).
A conventional rotary damper has a resistance part having an elliptical shape, so that air mixed into the housing during assembly is not allowed to be positioned between the resistance part of a rotor, i.e. a torque generating part, and a bottom surface or ceiling surface of the housing. However, because the rotor rotates in both directions, the air mixed into the housing generates a noise when the air moves over the resistance part to the opposite side of the resistance part. The noise generated when the air mixed into the housing moves over the resistance part is believed to be a bursting sound caused by the air mixed into the housing being compressed by moving up the resistance part and then being released suddenly when moving over the resistance part. Such a noise tends to happen more frequently when the viscous fluid has a higher viscosity, or when a distance between the rotor and the housing is narrower.
In view of the problem described above, an object of the invention is to provide a rotary damper, in which air mixed into a housing and compressed by moving up on a resistance part is gradually released during assembly, so that it is possible to prevent a noise caused by the air mixed into a housing even when the rotor rotates in both directions.
Further objects and advantages of the invention will be apparent from the following description of the invention.